This invention relates to the field of migration of seismic records, specifically reducing the cost of pre-stack migration of a plurality of seismic records by phase encoding and combining selected records before migration. As used herein, a seismic record includes shot gathers (a single source trace and multiple receiver traces), receiver gathers (a single receiver trace and multiple source traces), common offset gathers, and other combinations of source and receiver traces.
Complex subsurface conditions preclude simple stacking of seismic records. Complex subsurface conditions can scatter seismic waves in unexpected directions; simple stacking can show subsurface features in the wrong locations.
Migration of seismic records can provide more accurate location of subsurface features. Migration involves geometric repositioning of return signals to show an event (layer boundary or other structure) in its proper location. Pre-stack migration, or migration of seismic records before stacking into a single image, can provide the most accurate information.
Pre-stack migration techniques include Kirchoff migration, Stolt migration, finite-difference migration, Fourier finite-difference migration, phase-shift migration, Gazdag migration, split-step migration, phase shift plus interpolation, and reverse-time migration. See, e.g., Claerbout, Jon F., "Imaging the Earth's Interior", Blackwell Scientific Publications, Boston, 1985; Scales, John A., "Theory of Seismic Imaging", Samizdat Press, hilbert.mines.colorado.edu, 1994; Stolt, Robert H. and Benson, Alvin K., "Seismic Migration, Theory and Practice, Vol. 5", Handbook of Geophysical Exploration, Section I. Seismic Exploration, Geophysical Press, London, 1986; Yilmaz, O., "Seismic Data Processing, Investigations in Geophysics No. 2", Society of Exploration Geophysicists, P.O. Box 702740, Tulsa, Okla. 74170-2740, 1987.
All of these methods are expensive, however, especially when applied to seismic surveys comprising hundreds or thousands of seismic records. The expense can be mitigated by using post-stack migration, at the expense of accuracy in the final image. The expense can also be mitigated by using only subsets of the total number of seismic records, but at the cost of the information contained in the unused records.
Accordingly, there is a need for methods to reduce the cost of migrating seismic records, without losing information contained in the plurality of seismic records or reducing the image quality.